Methanol-to-olefins (“MTO”) processes, and more broadly, oxygenate-to-olefins (“OTO”) processes that use molecular sieve produce dimethyl ether (“DME”) as a byproduct. DME can be converted, in presence of certain molecular sieves, to olefins and is therefore recovered for recycle to the MTO reactor for such conversion. Several methods of isolating the DME from such olefins streams and delivering propylene useful in polymerization processes are known, such as in US 2005-0187358, US 2006-0111601, US 2008-0242908, WO 2006-061227 and WO 2003-033441.
In a fully optimized production train for producing polypropylene from oxygenates via an OTO (or MTO) reactor, it is advantageous to maximize the yield of propylene product while minimizing investment and operating costs. This must be accomplished without introducing DME to the polymerization reactor, and unfortunately increasing the yield of propylene product typically increases the amount of DME as a side product. Consequently, a simple combination of elements known in the art is not sufficient to produced propylene suitable for polymerization at a reasonable cost. What is needed is a method of further reducing investment costs in producing useful propylene, while maintaining the desired low level of oxygenates such as DME in the propylene.